The present invention relates to an artificial-reverberation generating device. Such a device is employed where a musical signal has little or no reverberation content of its own, but it is desired to make the signal sound as though it originated in an acoustic space having a desired degree of reverberation.
It is known to generate artificial reverberation by at least two methods. The first method involves the use of an algorithm in a digital signal processing technique using a combination of delay lines and filters to diffuse the “dry” (i.e. non-reverberant) input signal. The process can be defined with the aid of a small set of parameters such as reverberation time, damping, density (i.e. number of reflections in a given time), etc. As such, therefore, the reverberation characteristics can be readily set by the user. The second method is based on the idea of convolution. This is also a digital signal processing technique and involves the convolution of the dry input signal with the impulse response of an acoustic space. This is illustrated in FIG. 1, in which a convolution engine 100 is fed with impulse-response information from one or more impulse-response files 116. The impulse-response files 116 contain samples of actual reverberant environments. One way of realising the convolution engine is to use a finite impulse response (FIR) filter 114, whose length is the length of the desired impulse response. Alternatively, it is possible to transform the impulse response into the frequency domain to reduce the demands on processing power. Some methods employ a combination of time-domain and frequency-domain processing by splitting up the impulse response.
Both of these basic methods have drawbacks. In music production, artificial reverberation tends to be evaluated in terms that refer more to subjective taste than to objective technical properties of the sound involved. Terms often used in assessing reverberation are “smoothness”, “coloration” and “imaging”. Such terms can be translated into more technical language: smoothness may be considered to be equivalent to density, coloration to eigenfrequency distribution and imaging to phase correlation. The main drawback of the algorithmic method is that the eigenfrequency distribution, density and phase correlation are all dependent on the particular algorithm employed. The result is a compromise between all of these properties and the CPU resources that are available. As regards the convolution method, the user is limited to the specific impulse responses made available to him. If a short reverberation is needed, the user must have access to the impulse response of a small room. On the other hand, to create a long reverberation tail, the impulse response of a large room must be provided. It is possible to modify the envelope of an available room impulse response, but the results are far from satisfactory, mainly due to the low reverberation density that tends to characterize the start of a room response. With the convolution method, then, it is necessary to have available a large library of ready-made impulse-response files, and this increases costs.